I was thinking today, Google Maps has aerial views...
As it turns out, they have Streetview of Wales (or at least Cardiff) too. It's very weird to zoom in and see that big golden thing (which turns out to be the Wales Millennium Center) and the waterfall thing above the Hub. If you zoom in all the way, you can sort of make out the paving stone which is their secret exit.
I know it's not real, but if I went there, I would probably have a hard time not jumping on that stone and listening for a hollow sound.
I was unable to identify the other roof that Jack has been seen standing on, but there's a lot of Cardiff and the open square he was standing on is a relatively small detail. The Millennium Center is a lot easier to spot. If I watched that bit again and looked at the surrounding buildings I might be able to find it, but honestly I don't care that much.